In general, a branching device with the optical add-drop multiplexer (OADM) function is introduced into a WDM optical communication system. Conventionally, in order to change a network configuration in such optical communication system, paths and wavelengths in the branching device with the OADM function have to be reconfigured by replacing components such as optical filters.
In view of such circumstances, there has recently been an increasing demand for a branching device with the ROADM function, which allows for flexible reconfiguration of a network being already in operation. The ROADM function can be fulfilled by using a wavelength selective switch (WSS) that has splitting, switching, and combining functions. Devices provided with the ROADM function are disclosed in, for example, PTL 1 to PTL 4.
PTL 1 discloses an ROADM branching device based on a WSS. According to PTL 1, wavelength-division multiplexed signals inputted to the branching device are combined and subjected to wavelength selection in a WSS and then outputted.